Fuel cells generate an electric current in an electrochemical reaction between gaseous reactants, such as hydrogen and air. Fuel cells are different from other electrochemical devices, such as batteries, because fuel cells utilize active species that are stored external to the cells and these reactants are typically gas-phase reactants. In contrast, a traditional battery utilizes chemical energy in internally-stored active species in electrodes and electrolytes to generate electric current and the active species are typically either solids and/or liquids. The electrochemical reaction in some batteries can be reversed by applying an electric current, to recharge the battery.
One variation of a traditional battery is a flow battery, also known as a redox flow battery or redox flow cell. Unlike a traditional battery that has internally-stored active species, a flow battery utilizes externally-stored active species to generate electric current. The active species are typically dissolved in liquid electrolyte solutions. For example, the electrolyte solutions are circulated through the flow battery cells from external storage vessels. The active species are typically simple redox couples, such that the electrochemical reactions are reversible so that the flow battery can be recharged.
Another variation of a traditional battery is known as a metal-air battery, in which one electrode is a metal and the other electrode is air. Most metal-air batteries are not rechargeable. Rechargeable metal-air batteries require electrical reversal of the electrochemical reaction at a bi-functional air cathode catalyst, but such a catalyst is inefficient and the charging voltage is thus much higher than the discharging voltage, resulting in very low cycle efficiency, as well as the use of more expensive materials that do not corrode at high electrochemical potentials.